The invention is directed to a ceramic mass having a combination of a ceramic and a glass frit and to a capacitor having this ceramic mass.
Ceramic masses are known that are employed as a dielectric for multi-layer capacitors with metal electrodes. For cost reasons, copper is preferred as an electrode material. Given the employment of copper as the electrode material, however, it is necessary to reduce the sintering temperature of the ceramic mass below the melting temperature of the copper since the multi-layer capacitors are manufactured by a common sintering of the ceramic with the electrodes.
Technical solutions have already been disclosed that allow the common sintering of a ceramic mass with the Cu electrodes under reducing conditions, whereby the sintering temperature is lowered below the melting temperature of the copper (1083° C.). Specific sintering aids, preferably additives of the glass frits whose material basis is a system containing lead oxide and/or bismuth oxide, are utilized for this purpose. An oxygen partial pressure <10−2 Pa must be employed in order to suppress the oxidation of the copper during the sintering in the region of 1000° C. At the same time, a critical lower limit of the oxygen partial pressure dare not be downwardly transgressed since otherwise the ceramic or a constituent of the glass frit added in the manufacture is subjected to a reduction, which necessarily leads to a lowering of the insulation resistance and an inadmissible increase of the dielectric losses. In order to avoid a local downward transgression of this critical lower limit, the decarbonization of the green member employed in the manufacture must have been completely realized before the beginning of the sintering.
EP 0 534 802 A1, U.S. Pat. No. 5,479,140, U.S. Pat. No. 5,493,262, U.S. Pat. No. 5,488,019, U.S. Pat. No. 5,485,132 disclose ceramic masses of the material systems BaO—TiO2-(RE)2O3 wherein the oxide of the rare earth metals (RE) can be partially replaced by Bi2O3 and the sintering compression thereof already partially succeeding at 900° C. in that glass frit parts that contain CdO, PbO or Bi2O3 or glasses of the system ZnO—B2O3—SiO2 are added. This enables a common sintering with Ag electrodes in air. Compared to a partial reduction that results in a lowering of the insulation resistance and an increase in the dielectric losses, the systems prove insufficiently stable for a common sintering with the copper electrodes under inert conditions, for example in a nitrogen atmosphere.
EP 0 534 801 A1, U.S. Pat. No. 5,458,981 and U.S. Pat. No. 5,292,694 likewise disclose BaO—TiO2—SE2O3 ceramic masses in conjunction with the glass additives containing B2O3 and ZnO for the purpose of the common sintering with silver electrodes. In these instances, too, the decarbonization upon air admission prevents the combination with copper electrodes, so that recourse must be had to silver or silver/palladium alloys. The advantage of a cost-beneficial employment of the silver electrodes is opposed by the disadvantage of the high mobility of the silver, particularly at high temperature, that can lead to the migration effects and a deterioration of the dielectric properties resulting therefrom.
According to DE 197 49 858, the materials system BaO—PbO—Nd2O3—TiO2 used for the manufacture of the COG capacitors and microwave resonators with a high dielectric constant (DK) is tapped in the region of the phase formation of rhombic bronzes (Ba1−yPby)6−xNd8+2x/ 3Ti18O54 with 0.6 <x<2.1 and 0<y<0.6 for a sintering at temperatures <1030° C. and, thus, for the common sintering with the Cu electrodes in that the sintering aids, preferably PbO-free glass frits having a specific composition are added and a complete decarbonization in nitrogen is achieved due to the action of a water steam at an elevated temperature upon utilization of the steam reforming process known from crude oil processing. It must be noted as a limitation on this technical solution that the stability of the ceramic is limited by the PbO content, which requires an especially careful decarbonization and an extremely careful avoidance of too low an oxygen partial pressure. The two demands are linked to one another since, in particular, the inadmissible downward transgression of the critical oxygen partial pressure limit caused by the slight organic residual constituents must also be locally avoided. Otherwise, a eutectic Pb/Cu alloy is formed that melts at 954° C., which leads to an electrode run out.
Systems free of PbO and Bi2O3 have proven suited for avoiding such a disadvantage. DE 198 41 487 A1 disclosed the PbO-free materials system BaO—Nd2O3—Sm2O3—TiO2 in the region of the phase formation of the rhombic bronzes Ba6−x(SmyNd1−y)8+2x/3Ti18O54 for the manufacture of the COG capacitors and microwave resonators, whereby a temperature coefficient of the capacitance TKC<30 ppm/K or, respectively, a temperature coefficient of the resonant frequency TKν0<10 ppm/K is designationally set by means of a suitable selection of the composition parameters x and y, and a sintering at temperatures <1030° C. and, thus, a common sintering with the Cu electrodes is simultaneously achieved in that a glass frit with a suitable composition is added in an appropriate amount. The use of this advantage assumes that the rhombic bronzes of the appertaining composition are completely formed as a uniform phase before the sintering, which makes the relatively high conversion temperature of 1250° C. necessary in the calcination of the mixture of the oxide raw materials BaCO3, Nd2O3 and TiO2.